Differences between Lhasa apso and Shih tzu?

What are the differences between a Lhasa apso and a Shih tzu? How can you tell one apart from the other? They look basically the same to me, and I always confuse them...
And what about theit temperaments?
Thanks, Mariana

OH!!!!! I see! Looking at the muzzle and their bite makes it fairly easy. I would still love to hear other tips, but this really helped me.
I met a female shih tzu and a male lhasa apso. They live together and they really look like the same breed, so I got confused. The lhasa is a bit taller, but he has the same face than the shih tzu. He is really flat faced and has an undershot bite. He looks more like a pekingese than anything else (from the face.)...I guess he´s just atypical. I`ll try to post their pics so you can see them.
And yes...this Lhasa is more aloof, and the shih tzu is really playful.
Thanks again
Mariana